It is long been common to flow wastewater issuing from septic tanks of subsurface wastewater treatment systems to rows of structures buried in trenches within the soil, generally called leaching fields. Older leaching field systems comprise a crushed stone-filled trench with a lengthwise running pipe. More modern systems include prefabricated drainage units which are placed in trenches back-filled with stone and soil. One kind of such drainage unit comprises a generally cylindrical shape mass of plastic bead aggregate which is contained within in a sleeve of netting. Typically, a perforated pipe runs down the center of the mass of aggregate to distribute wastewater along the length of the drainage unit.
A familiar commercial example of such a unit is an EZflow® drainage unit, sold by Infiltrator Systems, Inc. The units are easily handled by installers and may be placed in straight or curved trenches. Typically, downward infiltration of soil or other overlying material into the interstices of the aggregate of a drainage unit is inhibited by use of a permeable geotextile fabric that is either laid on top of the unit after placement, or that has been attached to or integrated with the exterior of the unit. For instance, geotextile may be attached to the sleeve which retains the aggregate or may be a component of the sleeve. See U.S. Pat. No. 6,854,924 and U.S. Pat. No. 8,256,990 for examples. When such geotextile is part of a manufactured drainage unit the geotextile layer has been referred to as a barrier.
During use, wastewater flows outwardly from a drainage unit into the surrounding soil, sand or gravel. If the rate of inflow exceeds the rate of outflow into the surrounding soil, wastewater will be stored in the interstices of the aggregate. Much attention had been paid by designers, installers, repairmen and regulators to what happens at the interface between the drainage unit exterior surface and the surrounding soil or other medium. A so-called biomat is often observed at interfaces between a wastewater leaching unit and surrounding soil. When a drainage unit comprises a geotextile barrier, suspended solids in the wastewater can build up on the barrier, when they are too large to pass through the openings in the fabric of the barrier. That can inhibit the flow of water from the drainage unit into the surrounding soil. Thus, it has been considered by some that it is a disadvantage, or at least not necessary, to run a geotextile barrier all around the outer surface of a drainage unit, since geotextile is primarily needed to prevent infiltration of soil at the top, and to a lesser degree the sides. Generally, there is a continuing effort to enhance the performance of drainage units with respect to treatment of wastewater and avoiding degradation over time of use.